<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="box">
        <ul class="list">
            <li>
                <a href="">这是a</a>
            </li>
        </ul>
    </div>
    <script>
        // var a = document.getElementById("box").getElementsByClassName("list")[0].getElementsByTagName("li");
        // console.log(a); //[li,]
        // a[0]  标签对象  dom对象 

        var b = a[0].querySelector("a");
        console.log(b);
    </script>
</body>
</html>